1. Field of the Invention
This invention relates generally to patterned perpendicular magnetic recording media, such as disks for use in magnetic recording hard disk drives, and more particularly to patterned disks with data islands having improved magnetic recording properties.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In conventional continuous magnetic recording media, the magnetic recording layer is a continuous layer over the entire surface of the disk. In patterned media, also called bit-patterned media (BPM), the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. While BPM disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer, will likely be the choice for BPM because of the increased data density potential of perpendicular media. To produce the magnetic isolation of the patterned data islands, the magnetic moment of the spaces between the islands is destroyed or substantially reduced to render these spaces essentially nonmagnetic. Alternatively, the media may be fabricated so that there is no magnetic material in the spaces between the islands.
Nanoimprint lithography (NIL) has been proposed to form the desired pattern of islands on BPM disks. NIL is based on deforming an imprint resist layer by a master template or mold having the desired nano-scale pattern. The master template is made by a high-resolution lithography tool, such as an electron-beam tool. The substrate to be patterned may be a disk blank with the magnetic recording layer, and any required underlayers, formed on it as continuous layers. Then the substrate is spin-coated with the imprint resist, such as a thermoplastic polymer, like poly-methylmethacrylate (PMMA). The polymer is then heated above its glass transition temperature. At that temperature, the thermoplastic resist becomes viscous and the nano-scale pattern is reproduced on the imprint resist by imprinting from the template at a relatively high pressure. Once the polymer is cooled, the template is removed from the imprint resist leaving an inverse nano-scale pattern of recesses and spaces on the imprint resist. As an alternative to thermal curing of a thermoplastic polymer, a polymer curable by ultraviolet (UV) light, such as MonoMat available from Molecular Imprints, Inc., can be used as the imprint resist. The patterned imprint resist layer is then used as an etch mask to form the desired pattern of islands in the underlying magnetic recording layer.
In BPM, data are stored in an array of single-domain magnetic islands. The magnetic islands are required to have high coercivity (Hc) and be magnetically stable at room temperature. The figure of merit for the thermal stability of a magnetic island is given by the ratio (KuV/kBT) where Ku is the perpendicular magnetic anisotropy constant, V is the magnetic volume of the island, kB is the Boltzmann constant and T is the temperature. The contributions to Ku may include magneto-crystalline anisotropy and surface/interface induced magnetic anisotropy. The magnetic islands are considered thermally stable when (KuV/kBT) is typically above 80.
However, to provide higher bit areal density the magnetic islands lateral dimensions need to be smaller which leads to a lowering of V and thus a reduction in thermal stability. To compensate for the loss in V with higher areal density, Ku must be increased. In the case of a CoPtCr alloy magnetic material, increasing the Pt content and/or decreasing the Cr content can achieve higher Ku. In addition to high Hc and thermal stability the magnetic islands are also required to have a narrow intrinsic switching field distribution (iSFD), i.e., the island-to-island variation of the coercive field needs to be narrow enough to insure exact addressability of individual islands without overwriting adjacent islands. However, higher Pt content (e.g., greater than about 20 at %) leads to more stacking faults/defects and face-centered-cubic (fcc) phases, which affects the RL rocking angle, i.e., the angle of the c-axis of the hexagonal-close-packed (hcp) crystalline structure relative to the preferred perpendicular orientation, which is detrimental to a narrow iSFD. Also, lower Cr content can cause an increase in saturation magnetization, and therefore the magnetostatic energy is enhanced.
What is needed is a patterned perpendicular magnetic recording medium that has high Hc and Ku without the adverse effects on magnetic properties such as those caused by varying the amounts of Pt and Cr.